1. Field of the Invention
The present invention relates to an electric power steering system which uses an electric motor as a power source to reduce the force required to turn the steering wheel, and more particularly, to an electric power steering system including a clutch device for insuring a fail-safe operation in the event of an abnormality such as a blockage or locking of operation of the electric motor.
2. Description of the Relevant Prior Art
A conventional electric power steering system of this kind is disclosed, for example, in Japanese Patent Laid-open Publication No. 61-37581 published on Feb. 22, 1986. This system comprises a control device for controlling electric motor movement in response to a signal from a torque detecting device which detects a steering torque generated between an input shaft connected to a steering wheel and an output shaft connected to a steering gear or to a signal from a vehicle speed detecting device which detects the vehicle speed. The control device reduces the steering force of the steering wheel connected to the input shaft.
In this system, an electromagnetic clutch is controlled by the control device and is interposed between the electric motor and a reduction device. Thereby an improvement of the steering wheel feeling is obtained by controlling the ON-OFF condition of the electromagnetic clutch by the control device in response to the vehicle speed. Further, manual steering is possible by setting the electromagnetic clutch to the OFF condition in the event of an abnormality and fail-safe operation is insured. Furthermore, in this system, a mechanical clutch as another embodiment of the clutch device is interposed between the electric motor and the reduction device. This mechanical clutch is provided with a ball which is interposed between a cylindrical shaft connected to the electric motor and a six angle shaft connected to the reduction device and transmits the power of the electric motor to the output shaft via the reduction device and does not transmit the power from the output shaft to the electric motor side. According to this mechanical clutch, in the event of an abnormality of the electric motor, for example, the electric motor locks, manual steering is possible and fail-safe operation is insured.
In the above conventional electric power steering system of this kind, however, in the case of the electromagnetic clutch controlled to an ON or OFF condition by the control device, there is a danger in that operation of the electromagnetic clutch is impossible unless the clutch device and the control device operate erroneously. As a result, there is a danger that the steering wheel feeling deteriorates rapidly. For example, when the electric motor is not able to rotate, the electromagnetic clutch is not controlled to the OFF condition by the above causes except the clutch device itself and so there is the possibility that steering is impossible. Further, for example, when the electric motor is rotated toward the opposite direction against the normal rotation direction introduced to the input shaft by the above causes, the electromagnetic clutch is not controlled to the OFF condition and there is the possibility that steering is impossible. Furthermore, in this system, when the electric motor is not able to rotate due to the entry of a foreign substance into the system, since it is not able to rapidly detect the abnormality, the electromagnetic clutch is not controlled to the OFF condition and there again is the possibility that steering is impossible. And then, since the control device is unable to decide whether it means the end of steering or an abnormality in the electric motor, there is the possibility that traveling in the vehicle will be unsafe.
Also, on the other hand, in the case of the mechanical clutch, for example, when the abnormality is generated in the control device and the electric motor is rotated toward the opposite direction against the normal rotation direction introduced to the input shaft, the load of the electric motor is transmitted to the input shaft and there is the possibility that steering will be impossible. Similar to the above-described electromagnetic clutch, since the control device is not able to rapidly detect any abnormality, there is the possibility that traveling in the vehicle will be unsafe. Further, since this mechanical clutch is constituted by a freely rotating ball which is only interposed between the cylindrical shaft and the six angle shaft, the intermittent transmission of power between the electric motor and the reduction device is unstable. In particular, when a sudden steering movement is performed under a high load condition, since the electric motor also begins to rotate at high speed, once the ball moves into the narrow space sandwiching between the cylindrical shaft and the six angle shaft at high speed, thereafter there is a possibility that the ball is reposed toward the broad space by the shock. As a result, the mechanical clutch is controlled to the OFF condition rapidly and the force required for steering rapidly becomes difficult.
The above drawbacks are created because the above prior art clutch device is constituted to be intermittent in the transmission of power between the electric motor and the output shaft by the electric signal or to be one sided in the transmission of power from the electric motor to the output shaft. The prior art device does not control the intermittence of the transmission of the power between the electric motor and the reduction device mechanically in response to the rotation of the input shaft.
As mentioned above, in the above conventional electric power steering system of this kind, there is a drawback in which the safety performance and the reliability of the electric power steering system are not adequately insured.